#include<iostream>
#include<string>
#include<map> 
using namespace std; 

void solve(){
	string s;
	cin>>s;
	
	map<char,int> mp;
	for(int i=0;i<s.size();i++){
		mp[s[i]]++;
	}
	if(mp.size()==0){
		cout<<"a"<<endl;
		return ;
	}
	if(mp.size()==26){
		cout<<' '<<endl;
		return;
	} 
	for(int i='a';i<='z';i++){
		if(mp.count(i))continue;
			cout<<(char)i<<endl;
			break; 
	} 
	return;
}

int main(){
	int t=1;
	while(t--){
		solve();
	}
	return 0;
} 
